At that time it estimated the cost of the scheme would be about £3.5bn.

However, by last year the suggested price tag had risen considerably.

Some experts have suggested £15bn might be required for the project but others have said that £20bn would be a "conservative estimate".

A bridge too far?

Image copyrightGetty ImagesImage caption
The Hong Kong-Zhuhai-Macau bridge runs for 30 miles over water

In terms of distance - more than 20 miles for the Portpatrick project - it would not be the longest bridge over water in the world.

That honour goes, according to Guinness World Records, to the Hong Kong-Zhuhai-Macau bridge which has 48.3km (about 30 miles) of its span over water.

It credits the 36km (22.4 mile) Hangzhou Bay Bridge as the structure spanning the greatest expanse of open sea.

They are all dwarfed by the longest bridge of all - the Danyang-Kunshan Grand Bridge - at 164 km (102 miles) which stretches the furthest but not over water.

Could it be done?

Image copyrightGetty ImagesImage caption
The construction costs of major bridges, such as the Yangtze bridge, run into billions of pounds

According to the Channel 4 report, the prime minister wants to know "the risks around the project" - including "WW2 munitions in the Irish Sea".

This is nothing new as concerns have long been expressed about the area known as Beaufort's Dyke - a deep submarine trench where it has been estimated more than one million tonnes of weapons have been jettisoned.

Between logistics and costs, some have dismissed the scheme is unlikely to ever happen.

A senior Scottish government source has described the latest bridge proposal as "pie in the sky" and the "usual smokescreen bluster" from Boris Johnson.

However, one of the UK's leading architects Prof Alan Dunlop told Channel 4 News there would always be "naysayers".

"Such a project could be potentially tremendous for the country and show us as a forward looking and thinking country," he said.

He also told the BBC last year: "We do have incredibly talented architects and engineers in Scotland so I am sure that as a technical challenge it wouldn't be insurmountable."

Where there's a will?

Image copyrightMike FahertyImage caption
Larne has been suggested as one end of the huge construction project

Plans for some kind of link - either a tunnel or a bridge - go back as far as the 1890s but, despite much consideration, nothing has ever been built.

However, some political will has emerged in recent years.

In Northern Ireland, a proposal was contained in the 2015 DUP manifesto.

One MP told the BBC a bridge could make travel "less expensive and probably more certain".

Dumfries and Galloway Council said it was "certainly not averse" to the proposal being examined but felt there were other "more immediate priorities" for investment.

In particular, it highlighted the trunk roads network from the Cairnryan ports to the motorway network.

The Scottish government said this week that any such project would require close examination before it could even be considered.

'Practical obstacles'

"We are always keen to talk about how we can strengthen connections between Scotland, Northern Ireland and the Republic of Ireland," a spokesman said.

"There are an obvious number of practical obstacles and challenges to such a concept, and it would require a robust assessment of the costs or benefits of such a project in the first instance."

The UK government said that it regularly commissioned work to examine the feasibility of projects such as this.

"During the leadership campaign candidates spoke about a number of issues which resulted in Number 10 commissions ahead of a new prime minister taking over," it said.

"This PM has made no secret of his support for infrastructure projects that increase connectivity for people and particularly those that strengthen the Union."